This invention relates to a method and apparatus for rapidly characterizing photoresists which is fast, efficient and accurate.
Photoresists comprise coatings produced from solution or applied as a dry film which, when exposed to light of the proper wavelength, are chemically altered in their solubility to certain solvents (developers). Two types are used, negative acting resists and positive acting resists. A positive resist is insoluble in its developer until after exposure to light, while negative resists work the opposite way. Both resists define an image as a result of exposure to light.
Photoresists, particularly positive photoresists, have become very useful in the manufacture of printed circuits and semiconductors. The image quality depends on the optical transfer means (optical contrast) and the photoresist/developer system (chemical contrast). Photoresist material, however, can vary substantially in photoresponse and resolution capabilities depending on its age, source, and other factors. This can affect the quality of the photoresist image obtained, thereby affecting the quality of the product. It is consequently important for a manufacturer to know characteristic parameters for each batch of photoresist used.
One method used to determine response characteristics is by using characteristic curves. The standard way of drawing a characteristic curve is as a plot of resist thickness remaining versus log exposure. Using such a curve one can determine photospeed and contrast for different photoresist/developer systems with relative ease. Accordingly, the ease and accuracy with which such characteristic curves can be plotted is very important.
Characteristic curves have been plotted using step index exposures (small areas exposed with variable exposure energies on same substrate in wafer stepper) or step index type masks. These masks are placed over the photoresist and the system is then exposed to light. See. U.S. Pat. No. 3,631,772 of Curran et al. However, such masks only give a discrete number of points that can be plotted on the characteristic curve. Additionally, there is no assurance for a given mask that the most pertinent points will be obtained by the series of steps used. Therefore, a certain amount of trail and error testing using various masks may be necessary before a relevant characteristic curve can be obtained.
The method disclosed herein significantly simplifies the analysis procedure for photoresist material. This method also provides a more precise characteristic curve and consequently more accurate indication of a photoresist's photospeed and contrast.